Isocyanates are prepared in large volumes and serve mainly as starting materials for preparation of polyurethanes.
The production of isocyanates in the gas phase is associated with the formation of specific by-products that are solid or dissolved under the reaction conditions, for example isocyanurates, biurets, allophanates, carbodiimides or ureas.
Although these solid by-products—depending on the reaction conditions—form in low concentrations, they can lead to unpleasant deposition problems in specific process sections.
In the production of isocyanates in the gas phase, it has been found to be useful to contact the amine and phosgene reactants with one another by means of static mixing units, especially by means of nozzles. This can be accomplished, for example, by “jetting” the amine through a nozzle into a phosgene stream surrounding the nozzle. The inverse procedure (jetting phosgene into an amine stream surrounding the nozzle) is also conceivable, as is the use of two-phase nozzles for amine and phosgene. In each case, the problem arises that the gaseous amine and phosgene reactants come into contact with one another directly at the exit orifice of the nozzle. For this reason, the formation of the solid by-products begins there as well and can lead to deposits at the exit orifice of the nozzle.
These deposits at the exit orifice of the nozzle, under the conditions of the gas phase process, lead to additional vortexing and hence uneven radial distribution of the amine in the phosgene. These additional vortexes allow the reaction mixture to come into contact with the reactor wall at the early stage of the entry region of the reactor—with formation of solid deposits on the reactor wall. As a result, the free cross section of the reactor decreases ever further at the early stage of the entry region, with formation of an elevated pressure differential, which ultimately entails a premature shutdown for cleaning.
In the case of deposits at the exit orifice of the nozzle, in addition to the aspect of the shortening of the reactor service life, a worsened spectrum of by-products is observed since the additional vortexing at the exit orifice of the nozzle results in uncontrolled backmixing of reaction mixture. This gives rise to a dwell time profile in the reactor which is no longer exactly defined, and this has an unfavorable effect on the formation of by-products.
It is therefore desirable that, in the case of preparation of isocyanates in the gas phase, the phosgene and amine reactants do not come into contact with one another immediately at the exit orifice of the nozzle, but only at a certain distance therefrom.
Such a process variation would reduce the occurrence and hence also the deposition of solid by-products at the nozzle. In addition, such a process variation would homogenize the dwell time spectrum, which leads to a reduction in by-product formation.
WO 2009/027232 A1 details a method of metering an inert gas into a gap between the amine stream and the phosgene stream. It is stated therein that this additional metering of inert gas delays the main portion of the mixing of amine stream and phosgene stream in order to avoid the formation of solids at the nozzle tip.
However, a disadvantage of this method is that the inert gas added inevitably brings about cooling of the nozzle unless it—like the amine stream itself—is sufficiently overheated. However, no such overheating is disclosed in WO 2009/027232 A1.
The unpublished international patent application PCT/EP2015/071438 is concerned exclusively with a process for preparing pentane 1,5-diisocyanate (PDI).
In the case of inert gas-induced cooling of the nozzle, partial condensation of amine on the inner wall of the nozzle is likely. This is especially true in the case of an already elevated pressure differential through the reactor, which after a prolonged reactor run time already results in an elevated condensation temperature for the amine. Partially condensing amine would be discharged from the nozzle as droplets which in turn form deposits at the nozzle mouth after a certain time—as a result of thermal stress alone or else as a result of incomplete reaction with phosgene which is also present at the nozzle mouth in low concentrations.
There was therefore still a need for a simple and inexpensive modification of the process for preparing isocyanates in the gas phase with a sufficiently reduced tendency to deposition of solids at the nozzle, which avoids the specific disadvantages of the prior art processes.